ORIGIN OF BLOOD AND ENDOTHELIUM 241 



to the growing out onto the yolk of masses of cells from the 

 intermediate cell mass as described by Swaen and Brachet ('99, 

 '01). These cells in the species here studied may wander earlier 

 and more freely than in the trout. 



At any rate, as shall be brought out later, the ventral meso- 

 derm of the yolk-sac in other vertebrates and the intermediate 

 cell mass of the Teleosts are very closely related homologous 

 portions of the mesoderm, if not one and the same thing. 



4. The Jive-day embryos 



The conditions which the embryos have attained five days 

 after fertilization are illustrated in figures 1 to 4. In figure 1 

 a normal individual of this age is shown. The heart is slightly 

 twisted but stiU more or less tubular. The vascular network 

 on the yolk-sac is well established. The pigment cells are 

 numerous but not yet fully developed and have not assumed an 

 ahgnment along the blood vessels or taken on the usual embryonic 

 pattern. The heart, of course, is pulsating vigorously and the 

 blood current is easily seen both within the embryo and on the 

 yolk-sac. 



The three other figures in this group show different conditions 

 of arrested development in individuals without a blood circu- 

 lation. In figure 2 the pericardium is seen to be hugely dis- 

 tended so that the head is pushed or raised away from the yolk 

 surface and the heart is stretched out into a long narrow tube 

 extending from the ventral surface of the head to the sheer an- 

 terior surface of the yolk. This heart pulsates feebly and can 

 be seen to contain a small amount of fluid which is churned up 

 and down by the pulsations. None of this fluid, however, is 

 ever pumped away from the heart. The pigment is much less 

 plentiful than in the normal embryo of the same age, and the 

 individual chromatophores are smaller in size than those of the 

 normal embryo and have not sent out processes of any great 

 length. No blood vessels at all are seen within the yolk-sac 

 but very small scarcely noticeable blood islands are present 

 on the posterior yolk region though not indicated in the sketch. 



